Hi gisborne,
you seem pretty intent on bivvies, so Im not going to waste effort trying to convince you.
I got an old Australian Army version picked up from a friend. apparently he said its worth $400 new? (I think thats BS) weighed about a kilo, had a bug net and totally waterproof. made from authentic goretex material with a bug net . It was bug proof only if bugs didnt get in while you were getting in. once zipped up, it was bug proof. bugs outside couldnt get in, but unfortunately bugs inside couldnt get out either! a thermarest fits in with a sleeping bag just fine.
then i saw this Proforce bivvy from ebay
http://cgi.ebay.com.au/Pro-Force-Bivi-bag-bivvy-tent-shelter-military-army-NEW-/330440213031?cmd=ViewItem&pt=UK_SportsLeisure_HikingCamping_Tents_JN&hash=item4cefc58227 For the price and its claims, i really had to get one.
it really does weigh 310grams as stated, for only like $30-40. made out of abtex (simlar to goretex) and it really is totally water proof with its seam seals. it is however only slightly less breathable than the real goretex at 1/10 the price.it is NOT bug proof since there is no mesh, but i sewed a mesh ontop of it so now it is waterproof too. Not a wasted purchase since i plan to use this one inside my single wall tents to protect my sleeping bag and add some warmth.
If i was starting out again for a proper bivy, i'd go one of those bibler or outdoor research ones, or one of those newer bivvies made out of event.
I have some hardcore friends still in the military who sleep in nothing but bivvies but as for me, i like to move around in my shelter abit and cook, and get changed, and read, and keep my pack under a shelter etc.